A common method of making borosilicate nanoparticles is involves firing a mixture of boron and silicon containing oxides at a relatively high temperature(s) (i.e., 800° C.-1500° C.) to form the B—O—Si linkages.
Relatively low temperature (i.e., 50° C.-100° C.) sol-gel synthesis of borosilicates is also known wherein borosilicates are synthesized in the form of gels with Si—O—B linkages. However, these processes do not involve the separation of borosilicate particles as a separate entity, but are cast into borosilicate composite structures or coatings using the gel. A challenge in using a sol-gel process to synthesize mixed oxide nanoparticles is the difference in the reactivity of the precursors. For example, for borosilicate synthesis, the rate of hydrolysis and the subsequent oxide formation of boron precursors are several orders of magnitudes higher than that of the silica esters. This difference in the reactivity of the oxide precursors results in a physical mixture of individual oxides rather than cross polycondensed product, causing phase separation. To tailor the structure of the resulting materials it is necessary to control on the co-reactivity of the precursors.
Approaches to control the co-reactivity of two or more metal alkoxide species to avoid unnecessary phase separation include the use of chemical additives (e.g., glycols, organic acids (acetic acid), â-dicarbonyl ligands (ethyl acetoacetate (EACAC))) as chelating ligands to slow the hydrolysis and condensation reactions of non-silicate metal alkoxides. Although not wanting to be bound by theory, it is believed that after forming a complex with the chelating ligand, the species between metal and chelating agent is less easy to hydrolyze. However, the chelating ligand is believed to typically remain which alters the structure of the final network.
Another way to control the co-reactivity of two or more metal alkoxide species to avoid unnecessary phase separation is with chemically controlled condensation (CCC), wherein the hydrolysis of a fast-reacting alkoxide species is slowly initiated by the controlled release of water from the esterification of an organic acid with an alcohol. Once the fast-reacting alkoxide has been partially hydrolyzed and condensed, water is added to complete the overall reaction and to incorporate the slower reacting alkoxide.
Alternative methods for making borosilicate nanoparticles, preferably at lower temperatures, are desired.